Alfven-wave instability of current sheets in force-free collisionless plasmas

If the current sheet between adjacent twisted magnetic flux tubes is suffic iently thin, the electron flow velocity becomes comparable to the Alfven ve locity and can destabilize collisionless Alfven waves. The threshold for in stability in force-free plasmas is calculated for both inertial and kinetic Alfven-wave regimes. When there is strong magnetic shear, unstable kinetic Alfven waves can resonantly accelerate ions to energies much higher than t he electron temperature.